Double polysilicon gate structures are widely used in microelectronic fabrication owing to the inertness and stability of polysilicon at high temperatures (e.g. 1200.degree. C.). This property is useful in devices like CCDs where the electrodes have to be formed in the early steps of the process. Poly-ox, or oxide thermally grown from polysilcon, is widely used as a dielectric to insulate gate interconnects in two level polysilicon gate structures. Any weakness in the poly-ox is a potential reliability problem in devices such as CCDs that require two levels of polysilicon. The weakness in the dielectric properties of the poly-ox film is often related to the presence of a re-entrant profile formed during thermal oxidation of poly-1. This problem is illustrated in FIG. 1.
FIG. 1 shows a cross-sectional view of a double polysilicon gate structure. Layer 1 represents the Si substrate, layer 2 the gate oxide, layer 3 the first polysilicon gate or poly-1, layer 4the poly-ox, and layer 5 the second polysilicon gate or poly-2. The re-entrant profile is shown in circled region 6. It appears as a protruding lower edge in poly-2. Since electrical charge accumulates preferentially in edges and corners of a conductive material, the re-entrant profile provides an edge for charge build-up that can cause electrical shorts between poly-1 and overlapping poly-2 gate electrodes.
In U.S. Pat. No. 4,417,947 to Pan, the problem of the re-entrant profile is discussed and a method for the prevention of its formation is disclosed. However, this method requires the formation of a sloped poly-1 edge. A sloped poly-1 edge is not desirable if the first gate is to be used as an implant mask. For this application a vertical poly-1 profile is mandatory.